The linker cannot find an object that contains the method CRT uses to call your main. You may be trying to build a Console app using Windows application framework?! I suspect you may have picked/changed the project flags to a target which is not implemented by you.

Yeah, if you chose "Win32 Application" from the new projects dialog then you have to have WinMain() as you main function. If you're doing console it's just main() but you have to specify "Win32 Console Application" in the new project settings dialog. You can also change this in your project settings under the link tab by changing /subsystem:windows to /subsystem:console in the project options box.